Self-elevating marine platforms are used primarily for oil well drilling or production. Despite the many platforms that have been designed over the years, and the general similarity of various spaces, decks, tanks and their locations, the known platforms have certain limitations, chief among which are: (1) after one platform has been designed and constructed, the dimensions for the next contemplated platform cannot be readily varied, without essentially redesigning the first platform; (2) the preload tanks form integral part with the main hull thereby unduly limiting the choices of available deck space for quarters, machinery and cargo; (3) known platform designs do not readily lend themselves to modular construction for economic fabrication and to take advantage of shipyard availability; and (4) the location of the preload tanks precludes them from exerting optimum load during use and stability during tow.
The art of designing jackup platforms to meet the requirements laid down by the prospective owners and operators of such platforms is widely known. However, the known designs are limited to individual platforms and do not readily lend themselves to dimensional variations so as to take full advantage of the known designs.
To carry out the design operations systematically, the hull of a platform is divided into transverse frames. The volume between each pair of adjacent frames is called a "segment." The volume of each segment is computed together with the position of its center of volume. There may be more or less than 40 such segments for a typical platform.
To design a platform hull, it is necessary to maintain a running check of the segments' estimated weights and calculated bouyancy forces, and to keep track of the products of these weights and forces times the horizontal fore-and-aft distances or "moment arms." These products are known as the longitudinal weight and bouyancy moments. The forward-and-after moments of each segment are then computed in the same way as the fore-and-aft moments. The fore-and-aft positions of the centers of gravity of the individual segments are then estimated relative to a plane passing through the platform's mid-length. Separate sums are kept of the moments of these groups forward of and abaft the mid-length.
For the platform to float at a level attitude, the center of gravity and the center of bouyancy must lie in the same vertical transverse plane. It is possible to balance the weight between the platform's ballast tanks, which can be the preload tanks, until the desired attitude is attained. Such balancing, however, with prior art platforms is not easily attainable, especially if after the platform is built, the actual weights and volumes, or their centers do not agree exactly with the estimated values, as some machinery may have been added during the construction period.
The vertical position of the center of gravity is important in predicting the stability and behavior of a fully-jacked-up platform at sea under rough storm and wave conditions. This position may change by many feet depending upon the nature, amount, and disposition of the cargo and machinery. When the platform is elevated high out of water, it is at a disadvantage in winds and waves. It needs added mass to help it remain through waves and to reduce its tendency to overturn. Each preload tank is used to increase the total mass on the platform's legs and to place the center of gravity in a more advantageous position.
On the other hand, while being towed, the preload tanks become ballast tanks to help stabilize the platform in a seaway. The tanks can be filled with fresh water, or sea water and can be easily emptied when the weight is no longer desired. It is also desirable to have crew accomodations below the main deck. The main deck can then be used for cargo and machinery; and services and living quarters can be arranged in regions clear of preload tanks and cargo-handling areas.
Thus the arrangement, the disposition and the proportions of the structual materials in the hull, all known as the "configuration," are most important features.
When the structural configuration has been selected, conforming satisfactorily to the platform arrangement, the designer will select the scantlings, defined as the size, shape, area, and unit weight of the individual structural members. The preliminary scantlings are chosen from experience, from a platform generally similar, from classification society rules, or by an analytic process.
A platform hull with a deck, resists vertical and lateral bending and twisting all at the same time. Since some portions of the hull are in compression and shear from bending and twisting, the relatively thin bottom sides and deck must be prevented from buckling, crumpling, and wrinkling when the hull is strained. This is accomplished by the bulkheads which are reinforced with transverse and longitudinal stiffeners. The transverse and longitudinal bulkheads required for service, access, and storage, and the boundaries of internal tanks for liquids are utilized for structural strength wherever practicable.